As regards of SS isomer of general formula (I)
Where the * indicate asymmetric carbon atom having “S” configuration on both acid and alcohol moieties is of great interest, as the resolved isomer, esfenvalerate (SS) is biologically most active and has fourfold higher insecticidal activity than that of racemic fenvalerate a commercially important synthetic pyrethroid. An economically viable process for its preparation (SS isomer) is of great utility as it eliminates use of large quantity of a.i., thereby reducing chemical burden on environment.
Reference is made to EP Patent application 0,040,991 wherein a method for preparation of an enantiomeric pair (SS-RR) having insecticidal activity is described. The main draw-back of this process is that it yields a pair of enantiomers (SS-RR) instead of a single stereo isomer (SS).
Reference is made to the Patent application DE 2830031 wherein the process for preparation of esfenvalerate from its diastereomeric mixture is described. It describes a method for crystallization of SS isomer with or without using catalyst by four different processes.
The method A describes a process wherein SS isomer is crystallized without using a catalyst. The method B describes a process where in epimerisation at asymmetric carbon atom of alcohol moiety is effected by using a protic solvent or/and base catalyst. The method C describes a process wherein mother liquor after separating SS isomer is subjected to epimerisation with or without a catalyst. The method D performs the same procedure as in method C using a catalyst.
The main draw back of this process is that it employs protic solvents like methanol, ethanol with bases viz triethylamine, ammonia at temperature 0° C. to −5° C. At this temperature decomposition of the product, along with side reactions is observed. Another draw back is that the experimental conditions described are not completely reproducible albeit the procedure described is strictly followed.
EP patent 0050521, by Sumitomo Chemical Company Ltd. Japan, claims a method for preparation of SS isomer starting from a super saturated solution of diastereomeric mixture having respectively S configuration on acid moiety and (S), (R) configuration on Alcohol moiety using pure seed crystal solution of S-S isomer in presence or absence of basic catalyst. However the method could not be reproduced with consistency albeit the experimental conditions are followed as described in the patent.
Another drawback of this method is that when base catalyst like ammonia, triethylamine is used as described in experimental procedure of the patent, many a time product degeneration is observed in contrast to the crystallization of SS-isomer.
Preparation of (S)-α-cyano-3-phenoxy benzylalcohol by different methods viz. cyclic dipeptide, enzymes are known in prior art, reference is made to U.S. patent application Ser. No. 4,526,727 wherein a process is described for synthesis of (S)-α-cyano-3-phenoxy benzylalcohol using cyclic dipeptide, cyclo (D-phenylalanyl-D-histidine) and its subsequent esterification with (RS)-2-(4-chlorophenyl)isovaleric acid or its reactive derivatives to form a diastereomeric ester, followed by crystallization in a protic solvent to obtain SS isomer. The draw back of this process is that though cyclic dipeptide catalyst is used to synthesise the desired (S)-alcohol, the scale up operations for these processes are not yet commercially viable. Yet another draw back is that the process described for synthesis of the di peptide depend upon many physical characteristics like non-crystallinity, cohesiveness of the cyclic dipeptide which require stringent controls in preparation of the cyclic di peptide and are difficult to attain on scale up process leading to restriction of process parameters on a multi kilo scale preparation. Reference is made to U.S. patent application Ser. No. 5,177,242 where in a method to prepare optically active (S)-cyano-3-phenoxybenzylalcohol by reacting corresponding aldehyde with hydrocynic acid using an enzyme S-oxynitrilase immobilized on a membrane is described.
The draw back of this method is that though it may be performed with reasonable success on a laboratory scale, it is difficult to obtain the enzyme of desired activity, level on a multi scale operations as the enzyme activity is variant w.r.t. source of its isolation.
Another disadvantage of this method is the stringent physical parameters to be maintained for immobilization of enzyme on the membrane with out effecting denaturation may practically pose problems on a commercial process.